1. Field of the Invention
This invention relates to a semiconductor integrated circuit assembling method, particularly to an assembling method to effect wireless bonding.
2. Description of the Prior Art
Heretofore, the Mini Mod method has been proposed as an assembling method in which a semiconductor integrated circuit chip with bumps and a flexible film circuit substrate are combined. In the assembling method usually the bumps of the semiconductor integrated circuit chip are positioned to the leads of the flexible film circuit substrate respectively so as to be heated and pressured by means of a bonding tool and as the result, bonding is made. There have been proposed various combinations of the metal deposited at the upmost layer of the bump and the metal for surface treatment at the upmost layer of the lead. Among many methods, it is most popular to make the highest layer of the bump of gold, and the surface of the lead of tin, because its bonding characteristics are excellent.
However, many inconvenient problems arise when the Mini Mod method is applied to a timepiece and the flexible film circuit substrate is used as a circuit substrate of a timepiece. For example, it is very critical how the surface treatment of the flexible film circuit substrate is made in view of the combination of the bump and the lead. Where the bump is made of gold and the lead is made of tin, gold is far superior to tin in its corrosion resistance and long term stability, etc. when the other module components such as a crystal oscillating element, capacitor and display device of liquid crystal cells, etc. are soldered with each other to make electrically conductive connections. This is the reason why the so-called partially plating method is proposed in which the lead portion is made of tin, and the other fixture and contact portions are made of gold.
Further, there is proposed another method in which the flexible film circuit substrate is plated with tin, the semiconductor integrated circuit chip is bonded, the tin layer at the fixture and contacting portions of the module components are broken and then gold plating is effected.
However, these methods result in increased cost of the flexible film circuit substrate. Therefore, they are not preferable.
Where both the bump and lead made of gold, there is no problem in the reliability of the fixture and contacting portions at the module components, but the bonding pressure is increased due to the gold-to-gold junction of the bonding portions, the semiconductor integrated circuit chip is easily injured so that the yield is lowered and as the result, its cost is up. This is especially eminent in a digital watch that requires many pins on the semiconductor integrated circuit chip.